U.S. patent application number 16/896022 was filed with the patent office on 2021-12-09 for hard-to-intercept multiple coherent transmitter communications.
The applicant listed for this patent is Raytheon Company. Invention is credited to Benjamin P. Dolgin, Gary M. Graceffo, Andrew M. Kowalevicz.
Application Number | 20210384981 16/896022 |
Document ID | / |
Family ID | 1000004898998 |
Filed Date | 2021-12-09 |
United States Patent
Application |
20210384981 |
Kind Code |
A1 |
Dolgin; Benjamin P. ; et
al. |
December 9, 2021 |
HARD-TO-INTERCEPT MULTIPLE COHERENT TRANSMITTER COMMUNICATIONS
Abstract
An optical transmitter (and methods of transmitting and
receiving) includes a delay and modulation circuit (or
communications circuit) configured to receive at least one optical
beam and a data signal and generate at least two or more modulated
optical beams having the data encoded therein. One of the modulated
optical beams is a time-delayed or time-shifted version of another
one of the modulated optical beams, and both beams are directed
toward a target.
Inventors: |
Dolgin; Benjamin P.;
(Alexandria, VA) ; Kowalevicz; Andrew M.;
(Arlington, VA) ; Graceffo; Gary M.; (Burke,
VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Raytheon Company |
Waltham |
MA |
US |
|
|
Family ID: |
1000004898998 |
Appl. No.: |
16/896022 |
Filed: |
June 8, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 10/505 20130101;
H04B 10/541 20130101; H04B 10/5561 20130101 |
International
Class: |
H04B 10/50 20060101
H04B010/50; H04B 10/54 20060101 H04B010/54; H04B 10/556 20060101
H04B010/556 |
Claims
1. An optical transmitter comprising: an optical laser source
configured to output at least one optical beam; and a
communications circuit configured to: receive first data, receive
the at least one optical beam, transmit, using the at least one
optical beam, a first modulated optical beam encoded with the first
data in accordance with a predetermined phase modulation scheme,
and transmit, using the at least one optical beam, a second
modulated optical beam encoded with the first data in accordance
with the predetermined phase modulation scheme; wherein the second
modulated optical beam is a time-delayed version of the first
modulated optical beam; and wherein the communications circuit is
configured to apply a delay that renders the second modulated
optical beam incoherent with the first modulated optical beam at a
receiver.
2. The optical transmitter of claim 1, wherein the optical laser
source comprises a first laser source configured to output a first
optical beam and a second laser source configured to output a
second optical beam simultaneously with the first optical beam.
3. The optical transmitter of claim 2, wherein the communications
circuit comprises: a modulator configured to receive the first
optical beam and the second optical beam and generate the first
modulated optical beam and the second modulated optical beam; and a
delay element configured to generate the delay in the second
modulated optical beam in relation to the first modulated optical
beam.
4. The optical transmitter of claim 3, wherein the delay element is
disposed within an optical path associated with the second
modulated optical beam.
5. The optical transmitter of claim 3, wherein the delay element is
disposed within an electrical signal path associated with the
second modulated optical beam.
6. The optical transmitter of claim 1, wherein the delay is an
amount that is less than a symbol length of symbols transmitted
within the second modulated optical beam.
7. The optical transmitter of claim 6, wherein the delay is an
amount of time that is less than about 30% of the symbol length and
greater than a time equivalent of 100 wavelengths of transmission
for the second modulated optical beam.
8. The optical transmitter of claim 1, wherein the delay is an
amount of time greater than a time equivalent of 50 wavelengths of
transmission for the second modulated optical beam.
9. The optical transmitter of claim 3, further comprising: a
control circuit configured to vary the delay in the second
modulated optical beam.
10. The optical transmitter of claim 1, wherein the communications
circuit is further configured to generate, using the at least one
optical beam, a third modulated optical beam encoded with the first
data in accordance with the predetermined phase modulation scheme,
wherein the third modulated optical beam is another time-delayed
version of the first modulated optical beam.
11. A method of transmitting optical signals, the method
comprising: receiving first data; receiving at least one optical
beam; transmitting, using the at least one optical beam, a first
modulated optical beam encoded with the first data in accordance
with a predetermined phase modulation scheme; and transmitting,
using the at least one optical beam, a second modulated optical
beam encoded with the first data in accordance with the
predetermined phase modulation scheme; wherein the second modulated
optical beam is a time-delayed version of the first modulated
optical beam; and wherein the second modulated optical beam is
delayed by a transmitter so as to be incoherent with the first
modulated optical beam at a receiver.
12. The method of claim 11, wherein receiving the at least one
optical beam comprises: receiving a first optical beam from a first
laser source; and receiving a second optical beam from a second
laser source simultaneously with receiving the first optical
beam.
13. The method of claim 12, further comprising: modulating the
first optical beam to generate the first modulated optical beam;
modulating the second optical beam to generate the second modulated
optical beam; and generating the delay that causes the second
modulated optical beam to be the time-delayed version of the first
modulated optical beam.
14. The method of claim 13, wherein generating the delay comprises:
generating the delay within an optical path associated with the
second modulated optical beam.
15. The method of claim 13, wherein generating the delay comprises:
generating the delay within an electrical signal path associated
with the second modulated optical beam.
16. The method of claim 11, wherein the delay is an amount that is
less than a symbol length of symbols transmitted within the second
modulated optical beam.
17. The method of claim 16, wherein the delay is an amount that is
less than about 30% of the symbol length and greater than a time
equivalent of 100 wavelengths of transmission for the second
modulated optical beam.
18. A method of receiving optical signals, the method comprising:
receiving, at an etalon-based optical receiver, a first optical
beam modulated according to a specified signal, the first optical
beam carrying data and comprising phase modulations; receiving, at
the etalon-based optical receiver, a second optical beam modulated
according to the specified signal, the second optical beam carrying
the data and comprising phase modulations, wherein the received
second optical beam is a time-delayed version of the received first
optical beam; converting the received first optical beam into a
first intensity-modulated (IM) beam; converting the received second
optical beam into a second IM beam, wherein the first IM beam and
the second IM beam are incoherent; receiving, at a detector, the
first IM beam and the second IM beam; and generating and outputting
an electrical signal having a magnitude indicative of intensity of
the first IM beam and the second IM beam; wherein the received
second optical beam is time-delayed by a delay amount that is
greater than a time equivalent of 100 wavelengths of transmission
for the received second optical beam.
19. The method of claim 18, wherein the delay amount is less than
about 30% of a modulation rate of the received second optical
beam.
20. The method of claim 19, further comprising: receiving, at the
etalon-based optical receiver, a third optical beam modulated
according to the specified signal, the third optical beam carrying
the data and comprising phase modulations, wherein the received
third optical beam is another time-delayed version of the received
first optical beam; converting the received third optical beam into
a third IM beam; and receiving, at the detector, the third IM beam;
and wherein generating and outputting the electrical signal having
the magnitude indicative of the intensity of the first IM beam and
the second IM beam comprises: generating and outputting the
electrical signal having a magnitude indicative of intensity of the
first IM beam, the second IM beam, and the third IM beam.
Description
TECHNICAL FIELD
[0001] This disclosure is generally directed to laser communication
systems. More specifically, this disclosure is directed to
transmission of communications using an optical transmitter having
multiple coherent lasers.
BACKGROUND
[0002] Light waves may be made to carry information by modulating a
light source, such as a laser source, to change one or more of the
various properties of the light, such as amplitude, phase,
frequency, or wavelength. These light waves may be in the visible
spectral band, the infrared spectral band, or another region of the
electromagnetic spectrum. An optical receiver receives the light
waves and measures one or more properties or variations of the
light waves, such as amplitude, phase transitions, and the like,
from which the information may be recovered.
[0003] Conventional optical receivers for line-of-sight
communications using modulated light waves (such as modulated laser
beams) are configured to collect signals from a large area so that
the acquired signal power allows for accurate detection. Various
optics enable the capture and focus of light waves to concentrate
the signal power at a detector in the receiver. For some modulation
schemes, such as phase modulation, conventional receivers require
coherent light, so a laser is often used as the light source. When
such light is collected and focused, the best reception occurs if
all the light rays (across the cross-section of a telescope) arrive
at the detector in unison as a single wavefront, maintaining
alignment of the original phase relationships of the light rays.
When light rays have propagated through different media along the
way or are skewed, delayed, aberrated, or otherwise affected as is
typical for light waves traveling some distance through the
atmosphere, free space, or any other media, the light rays tend to
erode and ultimately destroy the coherency of the optical signal.
Without some form of wavefront correction in such systems, it is
difficult or impossible for conventional receivers to accurately
demodulate an incoming optical signal.
SUMMARY
[0004] This disclosure is directed to an optical/laser
communication system having multiple coherent laser transmitters,
each of which transmits the same data signal to the same
target.
[0005] In a first embodiment, an optical transmitter includes an
optical laser source configured to output at least one optical
beam. A delay and modulation circuit (or communications circuit or
circuitry) is configured to receive first data, receive the at
least one optical beam, and transmit, using the at least one
optical beam, a first modulated optical beam encoded with the first
data in accordance with a predetermined phase modulation scheme.
The delay and modulation circuit is also configured to transmit,
using the at least one optical beam, a second modulated optical
beam encoded with the first data in accordance with the
predetermined phase modulation scheme, wherein the second modulated
optical beam is a time-delayed version of the first modulated
optical beam.
[0006] In a further embodiment, the optical transmitter also
includes one or more phase or time delay elements configured to
inject a delay into an electrical path or optical path within the
transmitter resulting in the second modulated optical beam to being
delayed or time-shifted with respect to the first modulated optical
beam. In some embodiments, the first delay amount is an amount less
than a symbol length of symbols transmitted within the second
optical beam. Also, in some embodiments, the first delay amount is
an amount less than about 30% of the symbol length and greater than
100 wavelengths of the second optical beam.
[0007] In a second embodiment, a method of transmitting optical
signals includes receiving first data and receiving at least one
optical beam. A first modulated optical beam is transmitted, using
the at least one received optical beam, and encoded with the first
data in accordance with a predetermined phase modulation scheme. A
second modulated optical beam is transmitted, using the at least
one optical beam, and encoded with the first data in accordance
with the predetermined phase modulation scheme. The second
modulated optical beam is a time-delayed version of the first
modulated optical beam.
[0008] In a third embodiment, there is provided a method of
receiving optical signals including receiving, at an etalon-based
optical receiver, a first optical beam modulated in accordance with
a first signal, the first optical beam carrying data and comprising
phase modulations, and receiving, at the etalon-based optical
receiver, a second optical beam modulated according to the first
signal, the second optical beam carrying the data and comprising
phase modulations, and wherein the received second optical beam is
a time-delayed version of the received first optical beam. The
received first optical beam is converted into a first
intensity-modulated (IM) beam, and the received second optical beam
is converted into a second IM beam. A detector receives the first
IM beam and the second IM beam and generates and outputs an
electrical signal having a magnitude indicative of intensity of the
first IM beam and the second IM beam.
[0009] Other technical features may be readily apparent to one
skilled in the art from the following figures, descriptions, and
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] For a more complete understanding of this disclosure,
reference is now made to the following description, taken in
conjunction with the accompanying drawings, in which:
[0011] FIG. 1 illustrates an example laser-based communication
system having a multi-source (multi-laser) transmitter and an
etalon-based receiver in accordance with this disclosure;
[0012] FIG. 2A illustrates one embodiment of the transmitter shown
in FIG. 1 in accordance with this disclosure;
[0013] FIG. 2B illustrates another embodiment of the transmitter
shown in FIG. 1 in accordance with this disclosure;
[0014] FIGS. 3A, 3B, and 3C illustrate example time delays in
multiple optical signals emitted from the transmitter showing
correspondence to phase transitions between symbols in accordance
with this disclosure; and
[0015] FIGS. 4 and 5 illustrate methods of transmitting and
receiving, respectively, optical signals in accordance with this
disclosure.
DETAILED DESCRIPTION
[0016] FIGS. 1 through 5, described below, and the various
embodiments used to describe the principles of the present
invention in this patent document are by way of illustration only
and should not be construed in any way to limit the scope of the
invention. Those skilled in the art will understand that the
principles of the present invention may be implemented in any type
of suitably arranged device or system.
[0017] For simplicity and clarity, some features and components are
not explicitly shown in every figure, including those illustrated
in connection with other figures. It will be understood that any
features and components illustrated in the figures may be employed
in any of the embodiments described. Omission of a feature or
component from a particular figure is for purposes of simplicity
and clarity and is not meant to imply that the feature or component
cannot be employed in the embodiments described in connection with
that figure.
[0018] For purposes of this disclosure and as will be understood by
those skilled in the art, the terms "light," "light signal," and
"optical signal" may be used interchangeably and generally refer to
an electromagnetic signal that propagates through a given medium,
which may be empty space (such as a vacuum) or an atmospheric
medium (such as air). These terms are not meant to imply any
particular characteristic of the light, such as frequency,
wavelength, band, coherency, spectral density, quality factor,
etc., unless it is expressly stated or contextually clear that such
a characteristic is intended.
[0019] As described above, conventional optical receivers for
line-of-sight communications using modulated light waves are
configured to collect signals from a large area so that the
acquired signal power allows for accurate detection, and various
optics enable the capture and focus of light waves to concentrate
the signal power at a detector in the receiver. For some modulation
schemes, such as phase modulation, conventional receivers require
coherent light, so a laser is often used as the light source. When
such light is collected and focused, the best reception occurs if
all the light rays (across the cross-section of a telescope) arrive
at the detector in unison as a single wavefront, maintaining
alignment of the original phase relationships of the light rays.
However, when light rays have propagated through different media
along the way or are skewed, delayed, aberrated, or otherwise
affected as is typical for light waves traveling some distance
through the atmosphere, free space, or any other media, the light
rays tend to erode and ultimately destroy the coherency of the
optical signal. Without some form of wavefront correction in such
systems, it is difficult or impossible for conventional receivers
to accurately demodulate an incoming optical signal.
[0020] Thus, while it is generally desirable to maintain or recover
the coherency of a received optical signal or compensate for a lack
of coherency, this is often difficult to achieve. Some prior
approaches use adaptive optics to compensate for wavefront
variations caused by air perturbations (also known as
scintillation). Adaptive optics perform wavefront correction
directly on light rays and physically correct variations. However,
these approaches often have size and weight disadvantages. Also,
precise alignment of all elements of an adaptive optics system and
precise control of the adaptive optics are generally required for
acceptable operation and can be difficult to achieve.
[0021] This disclosure provides systems and methods for generation
and reception of phase-encoded (phase modulated) optical signals
without the need for a locally coherent clock source (meaning no
local laser or oscillator is needed at a receiver). In some
embodiments, an optical resonator, etalon (such as a Fabry-Perot
filter/resonator) or other functionally equivalent structure or
device, is used to convert a phase modulated optical signal into an
intensity-encoded optical signal. The intensity-encoded optical
signal may be used to detect information encoded in the phase
modulated optical signal. Various benefits can be achieved, at
least in part, using an optical front-end that includes an optical
resonator configured to detect modulation transitions, such as
phase variations, in a received optical signal without a coherent
reference source. The optical resonator further transforms the
modulation, such as the phase modulation, into an intensity
modulation that allows simplified processing, such as in the
electrical domain.
[0022] Examples of various systems for which demodulation of
phase-modulated optical signals may be useful or beneficial can
include communication systems, target designators, laser guidance
systems, laser sight, laser scanners, three-dimensional (3D)
scanners, homing beacons, and surveying systems. In at least some
of these examples, an optical signal is emitted and travels via a
free space signal path (known as free space optical or "FSO") to an
optical receiver. Although typically for use in free space
propagation, the features and components described here may be
utilized in other embodiments, such as those employing a fiber
coupling or another waveguide system. Systems and method for
demodulation of phase-modulated optical signals in accordance with
aspects and examples disclosed here may be applied to any of the
above example optical systems or other systems to generate,
transmit, receive, detect, and recover useful information from an
optical signal having phase encoding.
[0023] Those of ordinary skill in the art will understand that
optical signals modulated to carry information have one or more
characteristics that are changed by a transmitter in either a
continuous or discrete fashion or some combination of the two, and
segments of the light over time may be associated with the
particular characteristic(s) that indicate the information being
conveyed. For example, a phase modulated digital optical
transmitter may emit coherent light of a certain phase relationship
(relative to a reference time and/or phase) to indicate a
particular value. The light emitted to indicate the value may be
considered a segment or length of light whose phase indicates the
value. Later, the transmitter will alter the light characteristic
to emit a second segment of light to indicate a second value, then
a third segment of light, then a fourth segment of light, and so
on. As will be appreciated, a "symbol" is transmitted within each
segment, and the segment length is often referred to as the symbol
length. The rate at which the transmitter discretely alters the
characteristic, as in this example, is a modulation rate of the
transmitter, also known as a symbol rate. Each segment of light has
an associated physical length that is based upon the duration and
the speed of light in the propagation medium. For example, a
modulation rate of 10.sup.8 symbols per second (100 million
transitions per second) emits light segments of 10 nanosecond
duration with a length of approximately 3 meters. Higher modulation
rates generate shorter light segments, and lower modulation rates
generate longer light segments. Various embodiments of this
disclosure may operate at even higher transmission rates, such as 1
trillion transitions per second (1 Giga symbol/sec) or more. It
will be understood that a single light segment may have one of
multiple phase values (and possibly amplitude values), and
therefore the indicated value may be a multi-bit binary value
(symbol). Accordingly, modulation rate is not necessarily equal to
a transmission bit rate for a transmission system.
[0024] In addition to phase modulation of the emitted light, some
optical transmission systems may alter different or additional
light characteristics, such as amplitude, frequency, or wavelength,
and may also vary the modulation rate over time, such as based on
channel characteristics, noise, error rate, and the like.
Additionally, some optical transmission systems may modulate light
in an analog fashion, such as by a continuous variation in
amplitude of the light signal, and therefore not employ a
modulation rate per se. For purposes of this disclosure, aspects
and embodiments are generally described in the context of a
discrete transmission system including phase modulation, although
it will be understood that aspects and embodiments disclosed here
may be equally useful as transmitters and receivers for
transmission systems that generate light signals conveying
information differently than that described (such as a combination
of phase and amplitude modulations).
[0025] In addition, it will be understood that examples of methods
and apparatuses discussed herein are not limited in application to
the details of construction and the arrangement of components set
forth in the following description or illustrated in the
accompanying drawings. The methods and apparatuses are capable of
implementation in other examples and of being practiced or carried
out in various ways. Examples of specific implementations are
provided here for illustrative purposes only and are not intended
to be limiting. Also, the phraseology and terminology used here are
for purposes of description and should not be regarded as limiting.
Thus, for example, any references to front and back, left and
right, top and bottom, upper and lower, or vertical and horizontal
are intended for convenience of description, not to limit the
present systems and methods or their components to any one
positional or spatial orientation.
[0026] FIG. 1 illustrates an example laser-based communication
system 10 including a laser optical transmitter 100 and an
etalon-based receiver 150 in accordance with this disclosure. Note
that while the components of the optical transmitter 100 and the
optical receiver 150 identified in FIG. 1 (and other Figures) may
be shown and described as discrete elements in a block diagram and
may be referred to as "module," "circuitry," or "circuit," these
components may be implemented in any suitable manner. For example,
these components may be implemented as one or a combination of
analog circuitry, digital circuitry, or one or more processing
devices executing software instructions. In some embodiments,
components may be implemented using one or more microprocessors,
microcontrollers, digital signal processors (DSPs),
application-specific integrated circuits (ASICs), or field
programmable gate arrays (FPGAs). Unless otherwise indicated,
signal lines between components of the optical transmitter 100 and
between components of the optical receiver 150 may be implemented
as discrete analog, digital, or optical signal lines. Some of the
processing operations may be expressed in terms of calculations or
determinations performed by the optical transmitter 100, the
optical receiver 150, a detector, a controller, or other
components. The equivalent of calculating and determining values or
other elements can be performed by any suitable analog or digital
signal processing techniques and are included within the scope of
this disclosure. Unless otherwise indicated, signals may be encoded
in either digital or analog form.
[0027] In the illustrated embodiment, the optical transmitter 100
includes a delay and modulation circuit (or a communications
circuit or circuitry) 125a configured to receive one or more
optical beams from a laser source 116 and an input/data signal 110,
such as a digital bit stream. As will be appreciated, the delay and
modulation circuit (or communications circuit or circuitry) 125a
receives the input information and controls modulation of the one
or more optical beams to generate and output a plurality of
modulated or encoded optical beams 120a-120n each carrying the same
input information. Each of the beams 120a-120n are modulated in the
same manner and carry the same data. However, at least one of the
modulated optical beams 120a-120n is time shifted, or delayed in
time (time-delayed) with respect to another one of the modulated
optical beams. In other words, the timing or time position of the
data (or symbols) carried on one respective optical beam is shifted
relative the data (or symbols) carried on another respective
optical beam. This results in a time difference (or phase delay)
between when the symbol transitions occur in one respective optical
beam and when the same symbol transitions occur in the other
respective optical beam.
[0028] The one or more optical beams are controlled or modulated in
accordance with a desired encoding protocol established for
transmission of the information. In other words, one or more
control signal(s) are generated that, when applied to a modulator,
modulate or modify properties of the optical beam(s) in order to
carry the information. It will be understood that the input/data
signal 110 including the data may be in analog or digital form. In
addition, in one embodiment, the optical beams 120a-120n each have
essentially the same wavelength of light, while in another
embodiment, the optical beams 120a-120n have similar wavelengths.
Similar wavelengths are wavelengths that satisfy the equation
N .times. .lamda. i 2 = nL + .DELTA. .times. L i ##EQU00001##
where N is integer, nL is the optical length of the etalon (at the
receiver), .lamda..sub.i is the wavelength of the beam "i", and
values of |.DELTA.L.sub.i|/.lamda..sub.i are less within 5% of each
other, and in some embodiments within 1% or 2% from each other.
[0029] As will be described in further detail below and in other
figures, generating the modulated optical beams 120a-120n having
delay(s) with respect to each other may be implemented or
accomplished in different ways. In various embodiments, one or more
delay(s) may be added in electrical signal path(s), one or more
delays may be added in optical path(s), or in a combination
thereof.
[0030] One example of adding delay(s) in the electrical path may
include duplicating or splitting the input signal 110 (or signal(s)
generated after encoding) into a plurality of such signals, adding
delay(s) to one or more of the signals to produce a plurality of
time-shifted signals, and applying each signal to corresponding
modulators that generate the corresponding modulated optical beams
120a-120n.
[0031] One example of adding delay(s) in the optical path may
include applying the input signal 110 (or signal generated after
encoding) to a modulator to generate one modulated optical beam
which is then split or duplicated and an optical delay may be
inserted into one or more of the split optical beams to output the
corresponding modulated optical beams 120a-120n. Another example of
adding delay(s) in the optical path may include applying the input
signal 110 (or signal generated after encoding) to a plurality of
modulators (each receiving a separate optical beam) that generate a
plurality of modulated optical beams (identical with respect to the
timing of the data being carried), and then an optical delay may be
inserted into one or more of the modulated optical beams to output
the corresponding modulated optical beams 120a-120n. As will be
appreciated, numerous configurations and arrangements may be
utilized as desired to generate the modulated optical beams
120a-120n with at least one being time-delayed with respect to
another.
[0032] In the illustrated embodiment in FIG. 1, the optical
receiver 150 includes an optical resonator/etalon 160, one or more
optics 170, and a detector 180.
[0033] As will be appreciated, the optical receiver 150 is referred
to as an "etalon-based" receiver. An "etalon-based" receiver
includes an "etalon" which includes various devices and structures.
Note that the use of the term "etalon" and "resonator" throughout
this disclosure is not intended to be limiting and as used here may
include any of multiple structures, such as plates with reflecting
surfaces and parallel mirrors with various materials (which may or
may not include active optical materials) positioned in-between.
The spacing between the first semi-reflective surface and the
second semi-reflective surface of an etalon may be referred to as a
"cavity" but is not so limited. Optical resonators and etalons may
include other suitable structures, such interferometers and the
like. Additionally, resonator and etalon structures may be formed
as a laminate, layer, film, coating, or the like. This may include
Fabry-Perot etalons, micro-rings, optical delay line(s), optical
resonators or other types of resonators, which are configured to
sense variations, such as phase variations or modulations, in the
received optical signals 120a-120n. Accordingly, all of the
foregoing structures and devices are commonly referred to as an
"etalon" herein.
[0034] The etalon 160 may include one or multiple resonators or
etalons. In other embodiments, for example, each of the received
modulated optical signals 120 has a corresponding etalon 160. If
separate etalons 160 are used for different signals, different
optics 170 may be used with the different etalons 160, and
different detectors 180 may be used with the different etalons 160
and the different optics 170.
[0035] In some embodiments, the etalon 160 may be coupled to a pump
source (such as a laser), which may excite one or more components
(such as an active optical medium) of the etalon 160 to generate an
optical gain in the received optical signals 120a-120n. The
variations in each of the received optical signals 120a-120n are
representative of the modulation performed at the optical
transmitter 100. That is, the variations may be representative of
information encoded at the optical transmitter 100. The etalon 160
transforms the variations into an intensity modulation of output
optical signal energy, which are shown as output optical signal
energy 165a-165n in FIG. 1B. More specifically, the etalon 160
converts a phase modulation of each received optical signal
120a-120n in part by interaction of the arriving optical signal
120a-120n with resonant optical signal energy accumulated within
the etalon 160.
[0036] In some embodiments, the etalon 160 (or multiple etalons)
may include a pair of parallel semi-reflective surfaces with an at
least semi-transparent medium interposed therebetween. The
semi-transparent medium may represent an active optical medium that
provides an optical gain (such as an amplitude increase) when
excited by an optical or electrical signal. For additional examples
of optical resonators and etalons (and details thereof), reference
can be made to U.S. Pat. No. 10,313,022, which has been
incorporated by reference.
[0037] The etalon 160 may have one or more characteristic resonant
frequencies, each associated with a certain wavelength of light,
based upon the spacing (the optical length) between the
semi-reflective surfaces. In some embodiments, the surfaces are
semi-reflective and semi-transmissive, allowing some light through.
Accordingly, the arriving optical signals 120a-120n may be allowed
into the etalon 160 (between the pair of semi-reflective surfaces)
and may resonate inside the etalon 160 and between the pair of
semi-reflective surfaces. In addition, some of the resonating
optical signal energy inside the etalon 160 is emitted from the
etalon 160 through one of the semi-transmissive surfaces (referred
to as the "output optical signal energy"). The output optical
signal energy emitted from the etalon 160 is shown here as the
optical signal energy 165a-165n in FIG. 1.
[0038] The optical signals 120a-120n received at the etalon 160 may
establish a steady-state energy-preserving condition in which each
optical signal 120a-120n continuously arrives at the etalon 160,
accumulates or adds to build-up resonating optical signal energy
inside the etalon 160, and emerges from the etalon 160 at a
constant rate (meaning a steady-state output value). A variation in
the arriving phase and/or frequency of each of the optical signals
120a-120n will disrupt the optical signal energy resonating inside
the etalon 160 and, accordingly, disturb the corresponding output
optical signal energy 165a-165n. Once the steady-state condition is
re-established (meaning each respective optical signal 120a-120n
arrives at a constant rate without a variation), the respective
output optical signal energy 165a-165n returns to the corresponding
constant rate.
[0039] Accordingly, a change in phase, frequency, or amplitude of
the arriving optical signals 120a-120n causes a change in intensity
of the emerging output optical signal energy 165a-165n. A large
phase transition in the arriving optical signals 120a-120n, for
example, causes a large (but temporary) intensity change in the
emerging output optical signal energy 165a-165n. Similar operation
occurs in a micro-ring or other optical resonator. Accordingly, in
various examples, the etalon 160 functions as a demodulator or a
modulation converter for one or more received optical signals
120a-120n. Each of the emerging output optical signal energy
165a-165n therefore carries the same informational content as the
arriving optical signals 120a-120n, but in intensity-modulated
form.
[0040] The output optical signal energy 165a-165n is directed to
the detector 180 via the optics 170. The detector 180 converts the
emerging intensity-modulated output optical signal energy 165a-165n
into an electrical signal 185. In some embodiments, the detector
180 may include one or more photodetectors, such as one or more
photodiodes. In the embodiment shown, the detector 180 functions to
sum the intensity-modulated output optical signal energy 165a-165n
and output the electrical signal 185 (representing the total power
received at the detector 180. The electrical signal 185 may be
further processed by a digital or analog processing subsystem to
recover the data payload. One example of such a processing
subsystem is described in U.S. Pat. No. 10,313,022 and includes an
analog-to-digital converter and a digital processing subsystem
(which may function as a correlator and a code generator or which
may perform any other suitable processing). Reference is also made
to U.S. Pat. No. 10,305,602 (which is hereby incorporated by
reference in its entirety) for a description of a demodulator for
the demodulation of a QAM-modulated optical beam (having I and Q
components) using multiple etalons and various output responses of
etalons to phase transitions in the received optical signal.
[0041] As will be appreciated, in another embodiment (not shown),
the detector 180 may generate and output multiple electrical
signals that correspond to each of the optical signal energy beams
165a-165n received at the detector 180. These multiple electrical
signals may be further processed individually by a digital or
analog processing subsystem to recover the data payload, or
otherwise summed and then processed.
[0042] In some embodiments, the optical receiver 150 may include
additional or fewer optics than discussed above and may omit or add
various other components relative to those discussed above. For
example, the optics 170 may include focusing optics configured to
receive the emerging output optical signal energy 165a-165n from
the etalon 160 and to focus the output optical signal energy
165a-165n on the detector(s) 180. Also, the optical receiver 150
may include one or more optics that focus or otherwise collect and
direct the received optical signals 120a-120n to the etalon
160.
[0043] In some embodiments, the etalon 160 may include reflective
surfaces (including semi-reflective surfaces) that are not
co-planar and/or are not co-linear. For example, an interior
reflective surface of an etalon 160 may include some curvature, and
an opposing surface may also be curved such that a distance between
the two surfaces is substantially constant across various regions
of the etalon 160. In other embodiments, an etalon 160 may have
non-linear or non-planar surfaces with varying distances between
the surfaces at various regions and may still function as an
optical resonator or etalon for various wavelengths and at various
regions suitable for use in examples discussed here. Accordingly,
an etalon 160 may be purposefully designed to conform to a surface
or to have various regions responsive to differing wavelengths or
responsive to differing angles of arrival for a given
wavelength.
[0044] In some embodiments, the etalon 160 may be coupled to a pump
source, which may excite one or more components (such as an active
optical medium) of the etalon 160 to generate an optical gain in
one or more received optical signals. In other embodiments, the
etalon 160 may not include any pump source.
[0045] Now turning to FIG. 2A, there is shown one example (and more
detailed) embodiment of the optical transmitter 100 of FIG. 1,
denoted in FIG. 2A as optical transmitter 100a. As will be
understood, the optical transmitter 100a illustrates an optical
transmitter in which one or more delay(s) are incorporated or
inserted into an electrical signal path which results in the
desired modulated optical beams 120a-120n. For ease of reference
and example, this embodiment is illustrated with reference to
outputting at least three modulated optical beams 120a, 120b, 120n.
In other embodiments, the number of transmitted beams 120 may be
two or more.
[0046] In this embodiment, a delay and modulator circuit 125a is
shown as including a phase or time delay circuit 114a having delay
elements 140a-140n and a plurality of modulators 118a-118n--one for
each optical beam. Also in this specific embodiment, the laser
source 116 includes a plurality of laser sources 116a-116n each
outputting an optical beam to be modulated by the respective
modulator 118a-118n.
[0047] In one embodiment, the raw data or input signal 110 is
applied to an encoder 112 that generates one or more control
signals 115 for controlling, at a minimum, phase modulation of the
optical beam(s) by the modulators 118a-118n. In other embodiments,
the signal(s) 115 may include multiple signals each to control a
modulation of a different property of the optical beams, e.g.,
phase, amplitude, frequency, wavelength, etc. For example, the
control signal(s) 115 may include two signals--one for controlling
phase modulation and another controlling amplitude modulation.
Various combinations may be utilized. Depending on the specific
protocol, this may result in modulating the optical beam(s) to
carry information in a QAM-type modulation scheme (both phase and
amplitude) as ultimately applied to the optical beam. As will be
appreciated, other types of ultimate modulation schemes may be
used, such as n-QAM, phase-shift keying (xPSK), etc., and the
encoder 112 is not limited in generating the content and structure
of the control signal(s) 115 for controlling the modulators. It
will be understood that the specific protocol(s)--or specific
encoding and modulation scheme(s))--which describe the
correspondence between the transmitted information and the changes
in the optical beam properties are not further described, nor are
any further descriptions otherwise required for an understanding of
the concepts taught herein. As will be described below, those
skilled in the art will understand that benefits of the system
described herein can be achieved from detecting changes in phase
(or equivalent change in frequency).
[0048] The optical transmitter 100a includes a plurality of optical
sources 116a-116n, such as different laser sources, each for
generating and emitting a respective light/optical beam or wave
117a-117n (optical signals). Any number of optical sources
116a-116n may be utilized, as desired, including an embodiment in
which there is a laser source that generates single optical beam
117 that is split and applied to each of the modulator(s)
118a-118n. Each optical source 116a-116n emits the light beam
117a-117n that is modulated by a modulator 118a-118n to generate
and output the modulated optical signal or beam 120a-120n. As will
be appreciated, each modulator 118 may be configured to modulate
each respective light beam 117 in one or more ways.
[0049] In some embodiments, the optical sources 116a-116b output
light having the same wavelength or similar wavelengths. Each of
the optical sources 116a-116n is either aligned to or aimed at the
optical receiver 150, and the optical sources 116a-116n are
spatially disposed from each other. The actual spacing or
positioning of the optical sources 116a-116n may vary. In one
embodiment, an output structure 130, such as various optics, that
may include one or more optical lens or other optical transmission
components, may be used to direct the modulated optical signals or
beams 120a-120n towards a target, i.e., the optical receiver 150,
as needed.
[0050] Depending on the positional accuracy of the multiple lasers
116a-116n with respect to each other, as well as the layout
configuration of components and conductors in the transmitter, one
laser's output may be effectively time delayed or offset with
respect to another laser's output--even with no delay intentionally
added using one of the delay elements 140a-140n. Though largely
unintentional, such delay(s) may result in a significant time delay
between outputs thereby causing difficulty for conventional
receivers to receive/decode the received beams.
[0051] In one embodiment, the modulator 118 functions to phase
modulate (phase modulation) the light beam 117 in response to the
control signal(s) 115. It will be understood that, depending on the
magnitude of the phase change, that either the term "phase
modulation" or "frequency modulation" could be used. Although in
many cases, phase modulation is considered to be different from
frequency modulation, this patent document defines phase modulation
to include both phase and frequency changes, unless specifically
stated otherwise or is readily ascertainable from the context of
use. Thus, the term "phase modulation" as utilized herein is
intended to be broad and include equivalent changes in frequency.
Further, in other embodiments, one or more other techniques known
to those skilled in the art can also be applied or performed to
modulate a characteristic of the light beam 117, such as amplitude,
frequency, wavelength or variation in the modulation rate over
time.
[0052] The phase or time delay circuit 114a is configured to
receive the control signal(s) 115 and generate one or more
time-delayed control signal(s) 115a-115n--as shown. As will be
appreciated, each of the signals 115a-115n are duplicates or copies
of the original signal(s) 115, but are shifted in time with respect
to each other. In other words, other than being shifted in time (a
phase difference) with respect to each other, each of the signals
115a-115n correspond or match, or otherwise have essentially the
same waveform. Also, not each of the signals 115a-115n are required
to have different time shifts or differentials. In most
embodiments, at least one should be offset in time from another
one.
[0053] Each of the time-delayed control signal(s) 115a-115n is
input to each of the respective modulators 118a-118n to
individually modulate the respective light beams 117a-117n output
from the optical sources 116a-116n. The phase or time delay circuit
114a functions to intentionally inject a time or phase delay into
one or more of the replicated control signal(s). In some
embodiments, for example, the circuit 114 includes one or more
delay elements 140a-140n, where each delay element functions to
inject or generate a time delay of a predetermined specified amount
(which can be fixed, programmable, or otherwise variable).
[0054] The delay elements 140a-140n include any suitable structures
configured to delay an electrical signal by one or more fixed or
controllable amounts. In some embodiments, each of the delay
elements 140a-140n may represent separate components, such as when
each of the delay elements 140a-140n is implemented using separate
inverter chains or other sequential delay elements. In other
embodiments, the delay elements 140a-140n may be implemented using
a common structure, such as when all of the delay elements
140a-140n are implemented using a single inverter chain or other
sequential delay elements (with taps at given locations for desired
amounts of delay). Note that it may also be possible to implement
different delay amounts using different electrical
traces/conductors of different lengths. In general, this disclosure
is not limited to any particular technique or structures for
delaying an electrical signal by a fixed or controllable
amount.
[0055] In FIG. 2A, the transmitter 100a is shown having an optional
control circuit 210a (shown in block diagram form). Persons of
ordinary skill in the art will readily understand that any suitable
device(s), logic, or circuits (and methods) may be utilized, as
desired, to generate and output delay control signals for
controlling the delay elements 140a-140n to provide delay(s) in the
signal paths 115a-115n in accordance with the teachings herein. As
will be appreciated, the control circuit 210a may be controlled by
a processor (not shown) operating in accordance with one or more
software or firmware programs. The control circuit 210a may be
further configured to operate the transmitter 100a in one or more
modes of operation, as described below.
[0056] As described above, the phase or time delay circuit 114
receives the control signal(s) 115 and applies the signal(s) 115 to
each of multiple delay elements 140a-140n, which are optionally
controlled using one or more signals 211a from the control circuit
210a. Collectively, the delay elements 140a-140n output respective
delayed control signals 115a-115n, where each control signal
115a-115n has been delayed or shifted by the respective delay
amount. The delayed control signals 115a-115n are then applied to
the modulators 118a-118n which modulate the light beams 117a-117n
in accordance therewith, to generate the modulated optical signals
120a-120n. In another embodiment, for example, one of the control
signals 115a-115n may represent an undelayed version of the control
signal(s) 115, and no delay element may be used to generate that
version of the control signal.
[0057] The length of the delay(s) introduced by the circuit 114
will generally be less than a symbol length (length of symbol
encoded in the data signal 115), but substantially larger in
magnitude in terms of the wavelength(s) of the optical sources
116a-116n. The introduction of delays or time offsets in one or
more of the control signals 115a-115n effectively renders the
transmitted optical signals 120a-120n extremely difficult to
receive and demodulate in conventional receivers that utilize
adaptive optics and fiber, making them hard-to-intercept. It has
been found that when the time delays between the optical sources
116a-116n are small relative to the length of a transmitted symbol,
all symbols can be still recovered by the optical receiver 150
described herein. In other words, the injected time delay has
virtually no effect on reception and demodulation of the underlying
symbols and data. Therefore, multiple-laser transmissions can be
used in place of single-laser transmission, provided the total
power requirements are met or essentially the same.
[0058] Without the introduction of one or more delays, conventional
receivers utilizing adaptive optics and fiber (now or in the
future) will generally have the ability to receive and demodulate
the optical signals 120a-120n transmitted from the multiple optical
sources 116a-116n. In some embodiments, the transmitter 100a may
therefore enable or support two different modes of receiver
operation. A hard-to-intercept mode can be enabled when one or more
delays are added in one or more paths of the control signals
115a-115n. In this mode, the transmitted signals 120a-120n are
successfully received and demodulated using the optical receiver
150 in accordance with this disclosure. A normal mode can be
enabled when no delays are added. In this mode, although the
transmitted signals 120a-120n can still be successfully received
and demodulated using the optical receiver 150, a conventional
receiver using adaptive optics and fiber may also have the ability
to successfully receive and demodulate the transmitted signals
120a-120n.
[0059] The amount (or length) of the delay(s) may be fixed,
dynamic, programmable, variable, or involve any combination(s)
thereof. In other embodiments, the amount/length of the delay(s)
may vary randomly--which would necessarily increase the distortions
and incoherency of the transmitted light beams 120a-120n and render
reception by a conventional receiver even more difficult. These may
be random delay amount(s) in the signal path(s) and for random
period(s) of time.
[0060] In one embodiment, the laser-based communication system 10
utilizes multiple laser optical sources 116a-116n and
simultaneously modulates each of the multiple optical sources
116a-116n (with no intentionally-added delays in the encoded data
signal path) in essence according to the same encoded data signal
115. Prior systems that utilize a single laser source (single
channel) require all of the received optical power at a receiver to
originate from the single high-power laser source. In contrast, the
multiple laser optical sources 116a-116n in the laser-based
communication system 10 enables the use of lower cost low-power
lasers, where an additive effect is achieved by the etalon 160 when
receiving multiple versions of the same signal (the optical signals
120a-120n) from the multiple optical sources 116a-116n and
modulators 118a-118n.
[0061] In another embodiment, the laser-based communication system
10 utilizes multiple laser optical sources 116a-116n and modulates
at least one of the multiple optical sources 116a-116n with one of
the control signals 115a-115n and modulates at least another one of
the multiple optical sources 116a-116n with a delayed (or time
offset) version of the control signals 115a-115. In other
embodiments, any number of additional path(s) and delay(s) may be
provided, and delay amount(s) for all or some of the delay elements
140a-140n may be different, resulting in encoded data signals in
which at least one is offset in time from another one (or more). As
will be described further below, the inclusion of a substantial
delay in application of at least one of the control signal(s)
115a-115n to one or more of the light beams 117a-117n will render
the transmitted beams 120a-120n difficult to detect/receive/recover
by a conventional optical receiver--even when adaptive optics may
be utilized. Insertion of such a delay (or delays) causes
substantial wavefront distortion or mismatch (or incoherency) in
the transmitted beams 120a-120n that renders them difficult to
detect.
[0062] Although FIG. 1 illustrates one example of a laser-based
communication system 10 having a multi-source (multi-laser)
transmitter 100 (or 100a shown in FIG. 2A) and an etalon-based
receiver 150, various changes may be made to the system. For
example, the system 10 may include any suitable number of
transmitters 100, receivers 150, and/or transceivers incorporating
transmitters 100 and receivers 150.
[0063] It will be appreciated that only those components of the
optical transmitter 100, 100a needed to explain and understand the
concepts, methods, and systems disclosed herein are shown in the
FIGURES and described herein. Although not shown, the optical
transmitter 100, 100a may include various other components as
needed or desired. In some embodiments, the optical transmitter
100a may be configured in the same or similar manner as the optical
transmitter described in U.S. Pat. No. 10,313,022 (which is hereby
incorporated by reference in its entirety) and may further include
a forward error correction (FEC) module, a spreading module, a
mapping module, and/or a pulse-shaping filter. Additional optics
may also be included, such as one or more mirrors or lenses, which
direct each of the modulated optical signals or beams 120a-120n for
output. For example, the optics can be used to direct the modulated
optical signals 120a-120n in a direction of the optical receiver
150 via a signal path as shown in FIG. 1.
[0064] Note that while communication in FIG. 1 is shown as being
one-way from the optical transmitter 100 to the optical receiver
150, end devices may include both an optical transmitter 100 and an
optical receiver 150 (such as an optical transceiver) to support
bidirectional data communication. Each transceiver may be capable
of bidirectional data communication with another
transmitter/receiver pair.
[0065] Now turning to FIG. 2B, there is shown another example (and
more detailed) embodiment of the optical transmitter 100 of FIG. 1,
denoted in FIG. 2B as optical transmitter 100b. As will be
understood, the optical transmitter 100b illustrates an optical
transmitter in which one or more delay(s) are incorporated or
inserted into an optical signal path which results in the desired
modulated optical beams 120a-120n. For ease of reference and
example, this embodiment is illustrated with reference to
outputting at least three modulated optical beams 120a, 120b, 120n.
In other embodiments, the number of transmitted beams 120 may be
two or more.
[0066] The transmitter 100b of FIG. 2B includes various components
shown in the transmitter 100a of FIG. 2A and described above. As
will be appreciated, the transmitter 100b may form one component in
another embodiment of a laser-based communication system that also
includes the etalon-based receiver 150 illustrated in FIG. 1. One
obvious difference between the transmitters 100a and 100b is the
position and configuration of the phase or time delay circuit 114.
In transmitter 100a, the delay circuit 114a is disposed to provide
delay(s) in one or more electrical path(s) to perform electrical
path delaying, while in transmitter 100b, a delay circuit 114b is
disposed to provide delay(s) in one or more optical path(s) (after
the optical beams have been modulated) to perform optical path
delaying.
[0067] In one embodiment, the raw data or input signal 110 is
applied to the encoder 112 that generates the one or more control
signals 115 for controlling, at a minimum, phase modulation of the
optical beam(s) by the modulators 118a-118n--as similarly described
above with respect to optical transmitter 100a. Also, in the
specific embodiment illustrated, the optical transmitter 100b
includes a plurality of optical sources 116a-116n, such as
different laser sources, each for generating and emitting a
respective light/optical beam or wave 117a-117n (optical signals).
Any number of optical sources 116a-116n may be utilized, as
desired, including an embodiment in which there is a laser source
that generates a single optical beam 117 that may be split (e.g.,
optical splitter, not shown) and applied to each of the
modulator(s) 118a-118n. In the embodiment shown, each optical
source 116a-116n emits the light beam 117a-117n that is modulated
and output by the modulator 118a-118n to generate modulated optical
signals or beams.
[0068] One or more of the modulated optical signals or beams coming
from the modulators 118a-118n are input to a delay circuit 114b.
The phase or time delay circuit 114b functions to intentionally
inject a time or phase delay into one or more of the modulated
optical beams prior to transmission to a target. In some
embodiments, for example, the circuit 114b includes one or more
delay elements 141a-141n, where each delay element functions to
inject or generate a time delay of a predetermined specified amount
(which can be fixed, programmable, or otherwise variable).
[0069] The delay elements 141a-141n (optical delay lines or
elements) include any suitable structures configured to delay an
optical signal by one or more fixed or controllable amounts. In
some embodiments, each of the delay elements 141a-141n may
represent separate components, such as when each of the delay
elements 141a-141n is implemented using separate chains or other
sequential delay elements. In other embodiments, the delay elements
141a-141n may be implemented using a common structure, such as when
all of the delay elements 141a-141n are implemented using a single
chain or other sequential delay elements (with taps at given
locations for desired amounts of delay). Various structure(s) or
methods are known to, and may be implemented by, those of skill in
the art. In general, this disclosure is not limited to any
particular technique or structures for delaying an optical signal
by a fixed or controllable amount.
[0070] In FIG. 2B, the transmitter 100b is shown having an optional
control circuit 210b (shown in block diagram form). Persons of
ordinary skill in the art will readily understand that any suitable
structure(s), device(s), logic, or circuits (and methods) may be
utilized, as desired, to generate and output delay control signals
for controlling the delay elements 141a-141n to provide delay(s) in
accordance with the teachings herein. As will be appreciated, the
control circuit 210b may be controlled by a processor (not shown)
operating in accordance with one or more software or firmware
programs. The control circuit 210b may be further configured to
operate the transmitter 100b in one or more modes of operation, as
described above with respect to optical transmitter 100a.
[0071] In FIG. 2B, the phase or time delay circuit 114b receives
the modulated optical signals output from the modulators 118a-118n
applies one or more optical delays which are optionally controlled
using one or more signals 211b from the control circuit 210b.
Collectively, the delay elements 141a-141n output the respective
delayed modulated optical beams 120a-120n. In another embodiment,
for example, one of the modulated optical beams 120a-120n may
represent an undelayed version and no delay element may be used to
generate that version of the control signal.
[0072] The length of the delay(s) introduced by the circuit 114b
will generally be the same as the delay(s) introduced by the
circuit 114a in the embodiment shown in FIG. 2A. As will be
appreciated, the operational description, various embodiments,
teachings, and concepts applicable and described above with respect
to operation of the optical transmitter 100a above are also
applicable and may be applied to the optical transmitter 100b.
[0073] Although the embodiments of the transmitters 100a and 100b
are shown separately, a person of skill in the art will understand
that the two embodiments could be combined in whole or in part to
generate the modulated optical beams 120a-120n in which at least
one of the beams is time-shifted or delayed from another one of the
beams.
[0074] FIGS. 3A, 3B, and 3C illustrate example time delays in
multiple optical signals (such as signals 120a-120n) emitted from
the transmitter 100, 100a, 100b that correspond to phase
transitions between symbols in accordance with this disclosure. In
particular, FIGS. 3A, 3B and 3C illustrate an example propagation
of coherent light from the optical transmitter through a realistic
medium (such as air), where the light may encounter aberrations
(such as air perturbations). The light rays are influenced by air
perturbations or other obstructive influences that may affect a
portion of each of the signals 120a, 120b, 120n differently than
adjacent portions within each of the signals. As a result,
wavefronts 300a, 300b, 300n for the signals 120a, 120b, 120n may
become misaligned as illustrated.
[0075] As will be understood, the labeling of the wavefronts 300a,
300b, 300n in FIGS. 3A, 3B and 3C is arbitrary. Any position in
space and/or time of an optical signal may be identified as a
wavefront for purposes of discussing wavefront or phase alignment
with respect to other space-time positions. The phase relationship
or coherency of a bundle of light rays at one position in
space-time may change as the bundle of light rays propagates and is
influenced by the medium through which it travels. Also,
alterations in phase relationship experienced by a particular
bundle of light rays may not be the same as that experienced by
another bundle of light rays that come before or after. Therefore,
the alignment or misalignment of arriving wavefronts may change
significantly from one moment to the next, as illustrated by the
varying alignment shown for each wavefront 300a, 300b, 300n
illustrated.
[0076] When information carried by the optical signals 120a, 120b,
120n is contained in the phase of the signals, a conventional
optical receiver that would focus and concentrate the optical
signals 120a, 120b, 120n (such as an optic lens system) would
produce focused light that is not coherent and no longer carries
the phase information. To be able to retrieve the information, a
conventional optical receiver typically requires some form of
wavefront correction to restore the phase relationship across the
wavefronts 300a, 300b, 300n of the signals 120a, 120b, 120n. In
contrast, the optical receiver 150 described here is not affected
by these perturbations (delays on the order of less than a
wavelength).
[0077] Even if a conventional receiver employs a wavefront
correction mechanism, it cannot retrieve the carried information if
the three signals 120a, 120b, 120n are highly incoherent due to
different time delays incorporated into the signals 120a, 120b and
120n by either of the phase or time delay circuits 114a, 114b,
whichever method or structure may implemented. This is also
illustrated in FIGS. 3A, 3B and 3C. For illustrative purposes only,
in this explanation, assume that each wavefront is shown
corresponding to a phase change in the light beams (phase
transition between symbols). The distance between each of the
wavefronts 300a can represent a symbol length (or modulation rate)
within the signal 120a. Similarly, the distances between each of
the wavefronts 300b and between each of the wavefronts 300n can
also represent the symbol length within the signals 120b and 120n,
respectively.
[0078] An example of time or delay shifts of the wavefronts 300b
and 300n with respect to the wavefronts 300a are illustrated here.
As will be appreciated, to obtain these shifts or delays, the delay
element 140a may cause a delay (Delay a) equal to zero (no delay is
injected), the delay element 140b may cause a delay equal to an
amount Delay b, and the delay element 140n may cause a delay equal
to an amount Delay n. In one specific example, the Delay b may
equal about 10% of the symbol length, while Delay n may equal to
about 20% of the symbol length, and Delay a is equal to zero. Such
delays are typically on the order of hundreds and possibly
thousands of wavelengths of the lasers but are on the order of 0%
to around 30% of the symbol length. In other words, multiple
intensity-modulated beams with optically-large phase distortions
(hundreds to thousands of wavelengths) that correspond to
electrically-small phase delays (such as 30% or less of symbol
length) can be overlaid in a etalon-based receiver without
significant loss and the transmitted information can be
recovered.
[0079] In some embodiments, the predetermined delay amount is an
amount less than about 30% of a symbol length of symbols
transmitted within the second optical beam 120b. In other
embodiments, the predetermined delay amount is an amount between
about 5% and 20% of the symbol length and greater than 100
wavelengths of the wavelength of the second optical beam 120b.
[0080] By deliberating shifting/delaying either one or more of the
control signals 115a-115n with respect to each other (electric path
delaying) and/or one or more of the modulated beams after output
from the modulators (optical path delaying), the modulated output
signals 120a-120n are incoherent, which renders the signals
virtually undetectable by conventional receivers (when operating in
hard-to-intercept mode). By refraining from using any delays, the
output signals 120a-120n will be relatively coherent (aside from
wavefront variations resulting from propagation through air, etc.)
and may be detectable by a conventional receiver (when operating in
conventional mode).
[0081] FIGS. 3A, 3B, and 3C illustrate examples of time delays in
multiple optical signals emitted from multiple lasers where the
wavefronts correspond to changes in phase of the light from one
symbol segment to another symbol segment. This results in
detectable transitions between symbols. Various changes may be made
to FIGS. 3A, 3B, and 3C. For example, the wavefronts 300a, 300b,
300n here are examples only, and the actual wavefronts obtained
during use can vary widely based on a number of factors.
[0082] As will be appreciated, the optical transmitters 100a, 100b
can be actively controlled to switch from one mode to the other
mode as desired. For example, the respective control circuit 210a,
210b may include a controller or other logic circuitry for
controlling activation of the modes (and the associated amount of
each channel delay) based on user input or programming.
[0083] Although some examples illustrated and described above are
directed to injecting fixed electrical or optical path delays,
other embodiments may utilize programmable or variable time delays,
such as any other signal, pseudorandom modulation, or other
signals/timing that provide varying delays over time and per
signal. This may result in increased incoherency and therefore
increased difficulty in detecting the modulated optical signals
120a-120n on one hand, and may allow for increasing coherency by
proper adjustment of the encoded data signals and delays on the
other hand. In other words, the optical transmitters 100a, 100b may
be configurable to inject large phase/time delays in the optical
signals 120a-120n to achieve non-detectability while also having
the ability to configure the optical signals 120a-120n to appear
conventional (no delays) and be detectable by a conventional
receiver.
[0084] Now referring to FIGS. 4 and 5, there are illustrated a
method (400) of transmitting optical signals and a method (500) of
receiving optical signals in accordance with this disclosure and
teachings.
[0085] On the transmit end, data within an input signal (or data
signal) 110 is generated or received (step 410). A delay and
modulation circuit 125 receives at least one optical beam. (step
420). The delay and modulation circuit also receives the data
signal 110 and generates at least two modulated optical beams
120a-120n (using the at least one optical beam) each having the
data encoded therein (step 430). As described above, any suitable
type of phase modulation or encoding (including phase modulation
combined with amplitude or frequency modulation) may be utilized.
One of the modulated optical beams is delayed with respect another
by a delay amount. In other words, the encoded data (e.g., symbols)
of one modulated optical beam is delayed in time or phase with
respect the same encoded data of the other modulated optical beam.
The amount(s) of delay(s) may be less than a length of the symbols
transmitted on the optical beams 120a-120n and, in some
embodiments, is an amount less than about 30% of a symbol length
(and in some cases between about 5% and 25% of the symbol length)
and greater than 100 wavelengths of the laser wavelength. The
resulting modulated optical beams 120a-120n (with one or more
delays) are emitted and directed to a target (step 450).
[0086] To create the delayed version, two identical encoding
signals may be generated, and the delay is injected into the
electrical path of one prior to input to the modulators. These
original and delayed versions are applied to control the modulator
to generate undelayed and delayed modulated optical beams
120a-120n. In another embodiment, the delay may be injected in the
optical path of one optical beam after the optical beams have been
modulated by the same encoding signal.
[0087] On the receive end, an etalon-based optical receiver 150
receives the modulated optical beams 120a-120n with at least one
beam being a delayed or shifted version of another beam (step 510).
As will be appreciated, the transmitter and method used to generate
these received modulated optical beams 120a-120 may be irrelevant
to the receive end, as long as one constitutes a delayed or shifted
version of another as described or contemplated herein. One or more
etalons convert or demodulate the received optical beams 120a-120n
into intensity-modulated (IM) beam energy 165a-165n (step 520). The
IM beam energy 165a-165n is transmitted and focused via optics 170
on and received by one or more detectors 180, such as one or more
photodiodes or array of photodiodes (step 530). The detector also
converts the received optical energy into electrical signal(s) 185
(step 540). Since most, if not virtually all, of the optical energy
from each of the optical beams 120a-120n is received at the
detector 180, the detector 180 is configured to provide an additive
function to add the converted electrical signal(s) together (or
generates individual signals that can be added together) to
generate a signal indicative of the intensity of the received
optical beams (step 550). The converted electrical signal 185 can
then be further processed to determine or otherwise recover the
data represented by the detected phase changes in the optical beams
120a-120n (step 560).
[0088] In some embodiments, various functions described in this
patent document are implemented or supported by a computer program
that is formed from computer readable program code and that is
embodied in a computer readable medium. The phrase "computer
readable program code" includes any type of computer code,
including source code, object code, and executable code. The phrase
"computer readable medium" includes any type of medium capable of
being accessed by a computer, such as read only memory (ROM),
random access memory (RAM), a hard disk drive (HDD), a compact disc
(CD), a digital video disc (DVD), or any other type of memory. A
"non-transitory" computer readable medium excludes wired, wireless,
optical, or other communication links that transport transitory
electrical or other signals. A non-transitory computer readable
medium includes media where data can be permanently stored and
media where data can be stored and later overwritten, such as a
rewritable optical disc or an erasable storage device.
[0089] It may be advantageous to set forth definitions of certain
words and phrases used throughout this patent document. The terms
"application" and "program" refer to one or more computer programs,
software components, sets of instructions, procedures, functions,
objects, classes, instances, related data, or a portion thereof
adapted for implementation in a suitable computer code (including
source code, object code, or executable code). The term
"communicate," as well as derivatives thereof, encompasses both
direct and indirect communication. The terms "include" and
"comprise," as well as derivatives thereof, mean inclusion without
limitation. The term "or" is inclusive, meaning and/or. The phrase
"associated with," as well as derivatives thereof, may mean to
include, be included within, interconnect with, contain, be
contained within, connect to or with, couple to or with, be
communicable with, cooperate with, interleave, juxtapose, be
proximate to, be bound to or with, have, have a property of, have a
relationship to or with, or the like. The phrase "at least one of,"
when used with a list of items, means that different combinations
of one or more of the listed items may be used, and only one item
in the list may be needed. For example, "at least one of: A, B, and
C" includes any of the following combinations: A, B, C, A and B, A
and C, B and C, and A and B and C.
[0090] The description in the present application should not be
read as implying that any particular element, step, or function is
an essential or critical element that must be included in the claim
scope. The scope of patented subject matter is defined only by the
allowed claims. Moreover, none of the claims invokes 35 U.S.C.
.sctn. 112(f) with respect to any of the appended claims or claim
elements unless the exact words "means for" or "step for" are
explicitly used in the particular claim, followed by a participle
phrase identifying a function. Use of terms such as (but not
limited to) "mechanism," "module," "device," "unit," "component,"
"element," "member," "apparatus," "machine," "system," "processor,"
or "controller" within a claim is understood and intended to refer
to structures known to those skilled in the relevant art, as
further modified or enhanced by the features of the claims
themselves, and is not intended to invoke 35 U.S.C. .sctn.
112(f).
[0091] While this disclosure has described certain embodiments and
generally associated methods, alterations and permutations of these
embodiments and methods will be apparent to those skilled in the
art. Accordingly, the above description of example embodiments does
not define or constrain this disclosure. Other changes,
substitutions, and alterations are also possible without departing
from the spirit and scope of this disclosure, as defined by the
following claims.
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